Array waveguide diffraction grating type optical multiplexer/demultiplexer

ABSTRACT

An object of this invention is to cancel a polarization dependence of a transmission center wavelength due to the waveguide birefringence in an array waveguide and a slab waveguide in an array waveguide diffraction grating type optical multiplexer/demultiplexer filter applied for optical communication, particularly, wavelength split multiplexing system. In order to achieve the object of this invention, a polarization mode convertor converts the TE mode in a waveguide to the TM mode and the TM mode to the TE mode. This conversion function makes it possible to cancel the polarization dependence of the transmission center wavelength due to the waveguide birefringence in the array waveguide and the slab waveguide in front of and behind the polarization mode convertor.

TECHNICAL FIELD

The present invention is related to an optical multiplexer/demultiplexerapplied to optical communication. In particular, the present inventionis an optical multiplexer/demultiplexer which carries out multiplexingand demultiplexing of optical signals having different wavelengths in awavelength division multiplexing system, and is related to a diffractiongrating type optical multiplexer/demultiplexer which depends on an arraywaveguide.

PRIOR ART TECHNOLOGY

An optical multiplexer/demultiplexer which carries out multiplexing ordemultiplexing of optical signals having many different wavelengths isan essential requirement in a wavelength division multiplexing system.As for the kind of optical multiplexer/demultiplexer, an array waveguidediffraction grating type optical multiplexer/demultiplexer is mostlyused from the point of mass productivity and stability. A descriptionwill now be given for a prior art structure of an array waveguidediffraction grating type optical multiplexer/demultiplexer.

A silicon wafer is used as the waveguide substrate of the arraywaveguide diffraction grating type optical multiplexer/demultiplexer,and quartz glass is used as the waveguide material formed on top of thesilicon wafer. There is differential thermal expansion between thesilicon of the waveguide substrate and the quartz glass of the waveguidematerial, and because an internal residual stress is generated in theprocess of cooling to room temperature from the high temperature at themanufacturing time, a waveguide birefringence of about 0.0002 is createddue to the stress inside the array waveguide. This waveguidebirefringence makes the transmission center wavelength of the TM modewhich has an electric field perpendicular to the substrate shift towarda longer wavelength in comparison with the transmission centerwavelength of the TE mode which has an electric field parallel to thesubstrate. Namely, a wavelength shift is created due to the polarizationdependence of the transmission center wavelength. Hereinbelow, thedifference between the transmission center wavelength in the TM mode andthe transmission center wavelength in the TE mode is called apolarization wavelength shift. This polarization wavelength shift isapproximately 0.2 nm in an array waveguide diffraction grating typeoptical multiplexer/demultiplexer having a 0.4 nm demultiplexing gap.

In the prior art, a method (Japanese Laid-Open Patent Publication No.HEI 4-241304) of carrying out mutual conversion of the TE mode and theTM mode in which a polarization mode converter which depends on ahalf-wave plate having a main axis inclined at 45 degrees with respectto the substrate is inserted inside the array waveguide is proposed as amethod of canceling this polarization shift.

However, in this method, even though the polarization wavelength shiftbetween the center input/output ports can be canceled, strictly speakingthe polarization wavelength is different depending on the output port,and the polarization wavelength shift is not canceled at theinput/output ports away from the center.

SUMMARY OF THE INVENTION

The structure of an array waveguide diffraction grating type opticalmultiplexer/demultiplexer will now be described with reference to FIG.1. FIG. 1 is a schematic view of an array waveguide diffraction gratingtype optical multiplexer/demultiplexer. In FIG. 1, an input port 12 andan output port 22 are provided on a waveguide substrate 11, and an inputchannel waveguide 13, an input slab waveguide 14, an array waveguide 15,an output slab waveguide 18, output channel waveguides 21 and outputports 22 are sequentially connected from the input port 12. The outputports 22 have 64 ports from 22-1 to 22-64, and each channel waveguidethat forms the output channel waveguides 21 is connected respectively tothe output ports 22-1 to 22-64.

The plurality of channel waveguides that form the array waveguide 15 arearranged so that adjacent channel waveguides have path lengths which aredifferent by a prescribed value. The connecting portion of the inputchannel waveguide 13 and the input slab waveguide 14 and the connectingportion of the array waveguide 15 and the input slab waveguide 14 arearranged on arcs to mutually face each other, and in the same way theconnecting portion of the output channel waveguides 21 and the outputslab waveguide 18 and the connecting portion of the array waveguide 15and the output slab waveguide 18 are arranged on arcs to mutually faceeach other. The fact that phase delay differences of the optical signalsdue to the path differences of the array waveguide 15 are differentdepending on the wavelength is utilized, and the optical signals aredemultiplex into different output channel waveguides 21 in accordancewith each wavelength. As a result, the array waveguide diffractiongrating type optical multiplexer/demultiplexer has a wavelengthmultiplexing/demultiplexing function.

In FIG. 1, a groove 16 formed in the waveguide substrate 11 and ahalf-wave plate 17 are used for canceling the polarization wavelengthshift of the array waveguide 15. The half-wave plate 17 is inserted inthe groove 16 so that the main axis is inclined 45 degrees with respectto the waveguide substrate 11. By converting both polarizations of theTE mode and the TM mode with the half-wave plate 17, the polarizationwavelength shift of the array waveguide 15 before and after thehalf-wave plate 17 cancel each other out, whereby the polarizationwavelength shift of the array waveguide 15 is canceled.

FIG. 2 is a graph showing the polarization wavelength shift at eachoutput port of an array waveguide diffraction grating type opticalmultiplexer/demultiplexer which demultiplexes 64 waves with ademultiplexing gap of 0.4 nm. The polarization wavelength shift near thecenter output port number 32 is about −0.005 nm which almost cancels thepolarization dependence, but is approximately −0.03 nm at the outputport number 1 and approximately 0.03 nm at the output port number 64 atboth ends which forms a polarization wavelength shift of about 10% ofthe demultiplexing gap. At the output ports of these ends, because theintensity of the optical signals depends on the polarization, thiscauses the signal quality to be degraded.

The present inventors discovered the causes of different polarizationwavelength shifts which depend on the output port number, and thecanceling means thereof are described below.

First, from the results of an analysis carried out on the differentpolarization wavelength shifts which depend on the output port numberdescribed above, the cause of these kinds of polarization wavelengthshift was discovered to be due to the existence of about a 0.0007waveguide birefringence in the output slab waveguide 18. In FIG. 3, thefocusing state of optical signals having a polarization dependence isshown by light ray paths inside the output slab waveguide 18. In thisregard, the polarization dependence of the transmission centerwavelength due to the waveguide birefringence in the array waveguide 15is canceled by the previously mentioned technology.

Because a waveguide birefringence exists in the output slab waveguide18, the optical signals of the three wavelengths λ1, λ3, λ5 that shouldbe focused to the output ports 1, 3, 5 are focused in the TE mode asshown by the broken lines in the drawing, and focused in the TM mode asshown by the dotted lines in the drawing. Of these, the optical signalof the wavelength λ3 that should be focused to the center output port 3is focused in the same position in the TE mode and the TM mode withoutpolarization dependence. However, in the optical signals of thewavelength λ1 or the wavelength λ5 that should be focused at the outputport 1 or 5 of both sides of the output slab waveguide, it wasdiscovered that the focusing position will shift depending on thepolarization mode due to the waveguide birefringence of the output slabwaveguide 18, and the amount of this shift becomes larger as thefocusing position shifts to the ends. This means that the wavelength ofthe optical signal focused on a certain output port shifts depending onthe polarization, and the size of this polarization wavelength shiftbecomes larger as the position of the output channel waveguide moves tothe ends from the center.

Next, this fact will be described using mathematical equations. As shownin FIG. 3, the focal points of the optical signals are on an arc whichjoins the connecting portions of the output slab waveguide 18 and theoutput channel waveguides 21. When x is the distance along the arc fromthe focal point O at the center output port 3, the focusing position ofthe optical signal of the wavelength λ is given by:x=(na×ΔL−m×λ)×f/(ns×d)  (1)and the wavelength λ of the optical signal focused at the position wherethe distance is x from the above equation is given by:λ=(na×ΔL−ns×d×x/f)/m  (2)

In this case, na is the waveguide refractive index of the arraywaveguide 15, ns is the waveguide refractive index of the output slabwaveguide 18, ΔL is the path length difference of adjacent channelwaveguides of the array waveguide 15, m is the diffraction order, f isthe focal length of the output slab waveguide 18, and d is the spacingof the array waveguide 15 in the connecting portion with the output slabwaveguide 18.

It is understood from Equation (1) that if there is a polarizationdependence (birefringence) on the waveguide refractive index of theoutput slab waveguide 18 and the waveguide refractive index ns(TE) inthe TE mode and the waveguide refractive index ns(TM) in the TM mode aredifferent, then the optical signals of the wavelength λ will be focusedat different positions respectively in both polarizations. At the sametime, it is understood from Equation (2) that if ns(TE) and ns(TM) aredifferent, the wavelength focused at the position of the distance x willbe different in both polarizations. Using the difference between thewavelength λ(TE) in the TE mode and the wavelength λ(TM) in the TM mode,the wavelength difference due to this polarization, namely, thepolarization wavelength shift Δλ is defined as:

 Δλ=λ(TM)−λ(TE)  (3)

and this gives:Δλ=−Bs×d×x/(f×m)  (4)

In this case, Bs is the waveguide birefringence of the output slabwaveguide 18:Bs=ns(TM)−ns(TE)  (5)

It is understood from Equation (4) that as the demultiplexing numberbecomes larger and the position of the output waveguide shifts to theends from the center, the polarization wavelength shift Δλ increases inproportion to the distance x thereof.

In this way, it is possible to elucidate the cause of the polarizationwavelength shift which depends on the position of the output port asshown in FIG. 2. This means that the polarization wavelength shiftbecomes larger as the multiplexing/demultiplexing number of themultiplexer/demultiplexer becomes larger, and this becomes a big problemwhen the multiplexing/demultiplexing number of the opticalmultiplexer/demultiplexer is enlarged.

In view of the points described above, it is an object of the presentinvention to provide an array waveguide diffraction grating type opticalmultiplexer/demultiplexer which makes it possible to cancel the portdependence of the polarization wavelength shift caused by the waveguidebirefringence of the slab waveguide in the array waveguide diffractiongrating type optical multiplexer/demultiplexer.

In order to achieve the object stated above, the array waveguidediffraction grating type optical multiplexer/demultiplexer according toa first invention is an array waveguide diffraction grating type opticalmultiplexer/demultiplexer formed by at least one input channelwaveguide, an input slab waveguide connected to said input channelwaveguide, an array waveguide formed from a plurality of channelwaveguides connected to said input slab waveguide, an output slabwaveguide connected to said array waveguide, and at least one outputchannel waveguide connected to said output slab waveguide on thewaveguide substrate, wherein a polarization mode converter is providedin at least one of said input slab waveguide and said output slabwaveguide.

In the first invention, a description will be given for an operationwhich carries out demultiplexing of optical signals having a pluralityof wavelengths. The optical signals having a plurality of wavelengthsinputted from the input port propagate through the input channelwaveguide, and are branched by the input slab waveguide into a pluralityof channel waveguides which form the array waveguide. The arraywaveguide is constructed so that the path length is different betweenadjacent channel waveguides. The path length difference may be set at aprescribed value, or the path length difference may be set at a valueexpressed by a function. The connecting portion of the array waveguideand the output slab waveguide and the connecting portion of the outputslab waveguide and the output channels are arranged on arcs whichmutually face each other. Because the phase delay difference of theoptical signals created by the path length difference of the arraywaveguide is different for each wavelength, the optical signalsaccording to each wavelength are outputted to different output channelwaveguides. Namely, an optical demultiplexing function is achieved.However, because a waveguide birefringence exists in this output slabwaveguide, the polarization wavelength shift is different between outputports.

For this reason, a polarization mode converter is provided in the outputslab waveguide. Namely, the polarization mode converter has a functionwhich converts the TE mode to the TM mode, and the TM mode to the TEmode. As a result, the polarization dependence of the transmissioncenter wavelength created by the waveguide birefringence in the outputslab waveguide is canceled before and after the polarization modeconverter.

Accordingly, the polarization mode converter provided in the output slabwaveguide makes it possible to cancel the polarization dependence of thetransmission center wavelength created by the waveguide birefringenceexisting in the output slab waveguide, and this makes it possible tobuild an optical demultiplexer which suppresses the output portdependence of the polarization wavelength shift.

Further, with regard to the operation which carries out multiplexing ofsignals having a plurality of wavelengths in the first invention, it ispossible to think of the operation of demultiplexing the signals havinga plurality of wavelengths in reverse. The plurality of input ports areconnected respectively to corresponding input channel waveguides, andthe polarization mode converter is provided in the input slab waveguideconnected between the plurality of input channel waveguides and thearray waveguide. This polarization mode converter makes it possible tocancel the polarization dependence of the transmission center wavelengthcreated by the waveguide birefringence existing in the input slabwaveguide, and this makes it possible to build an optical multiplexerwhich suppresses the input port dependence of the polarizationwavelength shift.

Further, the optical multiplexer/demultiplexer in the first inventionwhich carries out multiplexing of signals having a plurality ofwavelengths and demultiplexing of signals having a plurality ofwavelengths can be thought of as a combination of an optical multiplexerand an optical demultiplexer. In this case, a polarization modeconverter is provided in both the input slab waveguide connected betweenthe plurality of input channel waveguides and the array waveguide, andthe output slab waveguide connected between the plurality of outputchannel waveguides and the array waveguide. These polarization modeconverters make it possible to cancel the polarization dependence of thetransmission center wavelength due to both the waveguide birefringenceexisting in the input slab waveguide and the waveguide birefringenceexisting in the output slab waveguide, and this makes it possible tobuild an optical multiplexer/demultiplexer which suppresses the inputport dependence and the output port dependence of the polarizationwavelength shift.

Further, the array waveguide diffraction grating type opticalmultiplexer/demultiplexer according to the second invention is theoptical multiplexer/demultiplexer of the first invention, wherein thearray waveguide formed from a plurality of channel waveguides carriesout means for canceling the polarization dependence due to the waveguidebirefringence.

There is differential thermal expansion between a silicon wafer used asthe waveguide substrate and silica-based glass used as the waveguidematerial formed on the silicon wafer, and this creates waveguidebirefringence inside the array waveguide. This generates a polarizationwavelength shift in the array waveguide, and because this degrades thesignal quality of the optical communication, the second invention isprovided with separate means for canceling the polarization dependenceof the transmission center wavelength due to the waveguide birefringenceof the array waveguide in addition to that of the input slab waveguideor the output slab waveguide. In this way, the polarization modeconverters provided in both the input slab waveguide and the output slabwaveguide are sufficiently designed to cancel only the polarizationdependence of the transmission center wavelength due to the waveguidebirefringence of both the input slab waveguide and the output slabwaveguide. Further, the polarization mode converter provided in eitherthe input slab waveguide or the output slab waveguide is sufficientlydesigned to cancel only the polarization dependence of the transmissioncenter wavelength due to the waveguide birefringence of either the inputslab waveguide or the output slab waveguide.

Because the second invention cancels the polarization dependence due tothe waveguide birefringence of the array waveguide by means separatefrom the means in the first invention, it is sufficient if only thepolarization dependence of the transmission center wavelength due to thewaveguide birefringence of both the input slab waveguide and the outputslab waveguide, or one of the input slab waveguide or the output slabwaveguide is canceled.

Further, the array waveguide diffraction grating type opticalmultiplexer/demultiplexer according to the third invention is theoptical multiplexer/demultiplexer of the first invention, wherein saidpolarization mode converter forms a means for canceling the polarizationdependence due to the waveguide birefringence of said array waveguide,and the polarization dependence due to the waveguide birefringence of atleast one of said input slab waveguide and said output slab waveguide.

Because the polarization mode converter provided in both the input slabwaveguide and the output slab waveguide, or in one of the input slabwaveguide or the output slab waveguide makes it possible to collectivelycancel the polarization dependence of the transmission center wavelengthdue to the waveguide birefringence of the array waveguide in addition tothat of the slab waveguides, there is no need to provide the arraywaveguide with a means for canceling the polarization dependence of thetransmission center wavelength due to the waveguide birefringence of thearray waveguide.

Accordingly, the third invention makes it possible to build an arraywaveguide diffraction grating type optical multiplexer/demultiplexerhaving a simple structure.

Further, the array waveguide diffraction grating type opticalmultiplexer/demultiplexer according to the fourth invention is theoptical multiplexer/demultiplexer of any one of the first, second orthird inventions, wherein said polarization mode converter is providedin a common groove formed in said input slab waveguide and said outputslab waveguide.

In the case where a polarization mode converter is inserted in both theinput slab waveguide and the output slab waveguide, there is the methodof forming a groove in each slab waveguide on the waveguide substrate,but in the fourth invention, the groove forming process is simplified byforming one groove in common in the input slab waveguide and the outputslab waveguide.

Accordingly, the fourth invention makes it possible to build an arraywaveguide diffraction grating type optical multiplexer/demultiplexermanufactured by a simplified forming process.

Further, the array waveguide diffraction grating type opticalmultiplexer/demultiplexer according to the fifth invention is theoptical multiplexer/demultiplexer of any one of the first˜thirdinventions, wherein said polarization mode converter is provided incommon in said input slab waveguide and said output slab waveguide in anintersecting portion formed by intersecting said input slab waveguideand said output slab waveguide.

In the case where a polarization mode converter is inserted in both theinput slab waveguide and the output slab waveguide, there is the methodof forming a groove in each slab waveguide on the waveguide substrate,but in the fifth invention, one groove is formed in common in the inputslab waveguide and the output slab waveguide, and one commonpolarization mode converter is provided in said groove in the input slabwaveguide and the output slab waveguide. This invention makes itpossible to simplify the forming process of the groove and eliminationof the polarization mode converter.

Accordingly, the fifth invention makes it possible to build an arraywaveguide diffraction grating type optical multiplexer/demultiplexer inwhich the forming process is simplified and the reduction of componentsis possible.

Further, the array waveguide diffraction grating type opticalmultiplexer/demultiplexer according to the sixth invention is theoptical multiplexer/demultiplexer of any one of the first˜fifthinventions, wherein said polarization mode converter is a half-waveplate having a main axis inclined at 45 degrees with respect to saidwaveguide substrate.

When the half-wave plate is inclined at 45 degrees with respect to saidwaveguide substrate, the light incident on the half-wave plate formsoutgoing light after the TM mode is converted to the TE mode, and the TEmode is converted to the TM mode. As described later, by using ahalf-wave plate as a polarization mode converter, there is the advantagethat it becomes possible to cancel the polarization dependence due tothe waveguide birefringence of the array waveguide in addition to thepolarization dependence due to the waveguide birefringence of the slabwaveguides.

Further, each of these inventions can be combined as mush as possible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an array waveguide diffraction gratingtype optical multiplexer/demultiplexer.

FIG. 2 is a graph showing the polarization wavelength shift at eachoutput port of an array waveguide diffraction grating type opticalmultiplexer/demultiplexer which demultiplexes 64 waves with ademultiplexing gap of 0.4 nm.

FIG. 3 is a principle drawing for describing the polarization dependenceof the focusing characteristics due to the waveguide birefringence ofthe output slab waveguide in an array waveguide diffraction grating typeoptical multiplexer/demultiplexer.

FIG. 4 is a schematic drawing for describing the structure of an arraywaveguide diffraction grating type optical multiplexer/demultiplexer inwhich the present invention is applied.

FIG. 5 is a principle drawing for describing the fact that it ispossible to cancel the polarization dependence of the focusingcharacteristics due to the waveguide birefringence of the output slabwaveguide by a polarization mode converter in an array waveguidediffraction grating type optical multiplexer/demultiplexer in which thepresent invention is applied.

FIG. 6 is a graph of measurement results showing the polarizationwavelength shift of each output port in an array waveguide diffractiongrating type optical multiplexer/demultiplexer in which the presentinvention is applied.

FIG. 7 is a schematic drawing for describing an embodiment of an arraywaveguide diffraction grating type optical multiplexer/demultiplexer inwhich the present invention is applied.

FIG. 8 is a schematic drawing for describing an embodiment of an arraywaveguide diffraction grating type optical multiplexer/demultiplexer inwhich the present invention is applied.

FIG. 9 is a schematic drawing for describing an embodiment of an arraywaveguide diffraction grating type optical multiplexer/demultiplexer inwhich the present invention is applied.

FIG. 10 is a principle drawing for describing the fact that it ispossible to cancel the polarization dependence of the focusingcharacteristics due to the waveguide birefringence of the output slabwaveguide and the array waveguide by a polarization mode converterprovided in the output slab waveguide in an array waveguide diffractiongrating type optical multiplexer/demultiplexer in which the presentinvention is applied.

FIG. 11 is a graph of measurement results showing the polarizationwavelength shift of each output port in an array waveguide diffractiongrating type optical multiplexer/demultiplexer before the presentinvention is applied.

FIG. 12 is a graph of measurement results showing the polarizationwavelength shift of each output port in an array waveguide diffractiongrating type optical multiplexer/demultiplexer in which the presentinvention is applied.

FIG. 13 is a schematic drawing for describing an embodiment of an arraywaveguide diffraction grating type optical multiplexer/demultiplexer inwhich the present invention is applied.

FIG. 14 is a schematic drawing for describing an embodiment of an arraywaveguide diffraction grating type optical multiplexer/demultiplexer inwhich the present invention is applied.

FIG. 15 is a schematic drawing for describing an embodiment of an arraywaveguide diffraction grating type optical multiplexer/demultiplexer inwhich the present invention is applied.

A description of the reference characters in the drawings is as follows.11 is a waveguide substrate, 12, 31-1˜31-64, 41-1˜41-17 are input ports,13, 32 are input channel waveguides, 14 is an input slab waveguide, 15,44 are array waveguides, 16 is a groove, 17 is a half-wave plate, 18 isan output slab waveguide, 19, 33, 42, 51, 53, 55, 58 are grooves, 20,34, 43, 45, 52, 54, 56, 57, 59 are half-wave plates, 21, 35 are outputchannel waveguides, 22-1˜22-80, 36 are output ports.

PREFERRED EMBODIMENTS OF THE INVENTION

The preferred embodiments of the present invention are described indetail below, but these embodiments are not interpreted as limitationson the present invention.

Embodiment 1

Descriptions of the preferred embodiments of the present invention aregiven below with reference to the drawings.

FIG. 4 shows an embodiment of an array waveguide diffraction gratingtype optical multiplexer/demultiplexer in which the second invention isapplied to the first invention. The special feature of the presentembodiment is a structure which cancels the polarization dependence ofthe transmission center wavelength due to the waveguide birefringence ofthe output slab waveguide by a half-wave plate inserted in the outputslab waveguide. A detailed description is given below with reference tothe drawings.

As shown in FIG. 4, the present array waveguide diffraction grating isconstructed by a silica-based waveguide on top of a waveguide substrate11 made of silicon, and is formed by an input channel waveguide 13 whichis connected to a input port 12, an input slab waveguide 14 which isconnected to said input channel waveguide 13, an array waveguide 15formed from a plurality of channel waveguides which are connected tosaid input slab waveguide 14, an output slab waveguide 18 which isconnected to said array waveguide 15, sixty four output channelwaveguides 21 which are connected to said output slab waveguide 18, andsixty four output ports 22 which are connected to said output channelwaveguides, whereby an optical multiplexer/demultiplexer having ademultiplexing number of 64 with a demultiplexing gap of 0.4 nm isformed.

As for the means of canceling the polarization dependence of thetransmission center wavelength due to the waveguide birefringence of thearray waveguide, up to now, the following methods have been disclosed:{circle around (1)} a method in which a groove is formed in both sidesof each channel waveguide of the array waveguide to release stressapplied to the array waveguide (E. Wildermuth et al.; ElectronicsLetters, Vol. 34, No. 17, p. 1661, 1998), {circle around (2)} a methodin which a thin film of amorphous silicon or the like is formed on thearray waveguide to adjust stress (Japanese Laid-Open Patent PublicationNo. HEI 5-157920), {circle around (3)} a method in which ultravioletlight is shown on the array waveguide, and the polarization dependenceof the refractive index change created thereby is used (M. Abe et al.;The 4th Micro Optics Conference and the 11th Topical Meeting onGradient-index Optical Systems (MOC/GRIN′ 93), Technical Digest, p. 66,1993), {circle around (4)} a method in which a polarization modeconverter which depends on a half-wave plate having a main axis inclinedat 45 degrees with respect to the substrate is inserted inside the arraywaveguide to convert the TE mode and the TM mode (Japanese Laid-OpenPatent Publication No. HEI 4-241304), {circle around (5)} a method inwhich a material such as germanium, silicon, boron or the like is addedto the silica glass of the waveguide material to approach the thermalexpansion coefficient of the silicon substrate (S. Suzuki et al.;Electronics Letters, Vol. 33, No. 13, p. 1173), {circle around (6)} amethod in which the fact that the waveguide birefringence is differentdepending on the waveguide width is used, and the width of the arraywaveguide is changed for each channel waveguide to adjust thepolarization dependence (Inoue et al.; Optical Fiber CommunicationConference (OFC) 2001, Technical Digest, WB-4, 2001), {circle around(7)} a method in which a birefringence plate is buried in a slabwaveguide, and the fact that the focused light beam passing therethroughis shifted depending on the polarization is used to cancel thepolarization dependence caused by the waveguide birefringence of thearray waveguide (Japanese Laid-Open Patent Publication No. HEI2000-292634).

Method {circle around (4)} described above is used in the presentembodiment. Namely, a groove 16 having a width of 18 μm and a depth of200 μm is formed to intersect the middle of the array waveguide 15, anda half-wave plate 17 made of polyimide having a thickness of 15 μm isinserted in the groove 16 with the main axis inclined at 45 degrees withrespect to the waveguide substrate 11 to form a polarization modeconverter. The polarization dependence of the transmission centerwavelength created by the waveguide birefringence of the array waveguideis canceled by this polarization mode converter. This is the same inEmbodiment 2.

Next, a groove 19 is formed to intersect approximately the middle of theoutput slab waveguide 18, and a half-wave plate 20 is inserted in thegroove 19 with the main axis inclined at 45 degrees with respect to thewaveguide substrate 11 to form a polarization mode converter. In thiscase, the half-wave plate 20 is made of polyimide and has a thickness of15 μm, and is inserted and fixed by an adhesive in a groove 19 having awidth of 18 μm and a depth of 200 μm processed to intersectperpendicularly with respect to the optical axis of the output slabwaveguide 18. Further, optical fibers are connected to the input port 12and the output ports 22, and these elements are assembled together withessential elements such as a heater, a Peltier element and the like in acasing to form a module, whereby the array waveguide diffraction gratingtype optical multiplexer/demultiplexer of the present invention ismanufactured.

FIG. 5 is a drawing showing the state where the output port dependenceof the polarization shift is canceled by the polarization mode converterconstructed by the half-wave plate 20 made of polyimide inserted inapproximately the middle of the output slab waveguide 18 in the arraywaveguide diffraction grating type optical multiplexer/demultiplexerused in the present invention.

The optical signals of the three wavelengths λ1, λ3, λ5 that should befocused at the output ports 22-1, 22-3, 22-5 receive the effect of thewaveguide birefringence of the output slab waveguide 18 in accordancewith the polarization of the TE mode and the TM mode at the arraywaveguide 15 side from the half-wave plate, and focusing is begun asshown by the dashed or dotted lines, respectively. Next, after passingthrough the polarization mode converter formed from the half-wave plate20 made of polyimide, the optical signals of the TE mode are convertedto the TM mode, and the optical signals of the TM mode are converted tothe TE mode, and thereafter each of these receives the oppositewaveguide birefringence effect up to that point and propagates. As aresult, the optical signals of the TE mode and the TM mode of thewavelengths λ1, λ3, λ5 are finally focused at the same position for eachwavelength without relation to the polarization. Namely, the output portdependence of the polarization wavelength shift is canceled by thepolarization mode converter provided in the output slab waveguide.

The distribution of the polarization wavelength shift among the outputports in the present embodiment is shown in FIG. 6. In FIG. 6,cancellation was carried out up to the point where the polarizationwavelength shift among the output ports was approximately zero, and evenat the output port number 1 and the output port number 64 at both endsof the output slab waveguide 18, the polarization wavelength shift wasreduced to within ±0.005 nm.

From the above, it was understood that the polarization mode converterwhich depends on the half-wave plate 20 inserted in approximately themiddle of the output slab waveguide 18 carries out satisfactoryoperation in canceling the output port dependence of the polarizationwavelength shift, and the array waveguide diffraction grating of thepresent embodiment achieves satisfactory characteristics as an opticaldemultiplexer.

The reason a half-wave plate made of polyimide is used in thepolarization mode converter inserted in the slab waveguide in thepresent embodiment is because it is possible to reduce the loss of thesignal light due to the insertion of a half-wave plate when thethickness of the half-wave plate is made thin to 20 μm or smaller, andbecause it is especially suited as a polarization mode converterinserted in a waveguide as in the present embodiment. A polarizationmode converter which depends on this half-wave plate made of polyimideis used in the prior art where it is inserted inside the array waveguideto cancel the polarization dependence of the transmission centerwavelength due to the waveguide birefringence of the array waveguide,but up to now there is no example of this being applied to cancel thepolarization dependence of the transmission center wavelength due to thewaveguide birefringence of the slab waveguide, and in the presentembodiment it is clear that this functions effectively as a polarizationmode converter for canceling the polarization dependence of thetransmission center wavelength due to the waveguide birefringence of theslab waveguide, namely, the output port dependence of the polarizationwavelength shift.

Embodiment 2

FIG. 7 shows a second embodiment of an array waveguide diffractiongrating type optical multiplexer/demultiplexer in which the secondinvention is applied to the first invention. The special feature of thepresent embodiment is a structure which cancels the polarizationdependence of the transmission center wavelength due to the waveguidebirefringence of the input slab waveguide by a half-wave plate insertedin the input slab waveguide. A detailed description is given below withreference to the drawings.

As shown in FIG. 7, the present array waveguide diffraction grating isgiven approximately the same structure as the first embodiment exceptfor the fact that there are sixty four input ports 31 and one outputport 36, the fact that a half-wave plate 34 is inserted as apolarization mode converter in the input slab waveguide 14, and the factthat a half-wave plate is not inserted in the output slab waveguide 18.

Because the array waveguide diffraction grating of the presentembodiment is given a structure in which the input and the output of thefirst embodiment are switched, it has a function in which the input andoutput of the first embodiment are reversed. Namely, the presentembodiment functions as an optical multiplexer having a multiplexing gapof 0.4 nm and multiplexing number of 64. The polarization mode converterinserted in the input slab waveguide 14 mutually converts the TE modeand the TM mode in the same way as the polarization mode converterinserted in the output slab waveguide 18 in the first embodiment, andcancels the input port dependence of the polarization wavelength shiftcaused by the waveguide birefringence of the input slab waveguide 14.

In the present embodiment, a structure in which a half-wave plate 34made of polyimide having a thickness of 15 μm is inserted in a groove 33formed in approximately the middle of the input slab waveguide 14 sothat the main axis is inclined at 45 degrees with respect to thewaveguide substrate 11 is used as a polarization mode converter. Thesame measurements as those of FIG. 6 were carried out, and from themeasurements of the input port dependence of the polarization wavelengthshift for the array waveguide diffraction grating type opticalmultiplexer of the present embodiment, it became clear that the inputport dependence of the polarization wavelength shift is almost canceled.Namely, the polarization wavelength shift is within ±0.005 nm for allthe input ports, and it is understood that this makes it possible toachieve satisfactory characteristics as an optical multiplexer.

Embodiment 3

FIG. 8 shows an embodiment of an array waveguide diffraction gratingtype optical multiplexer/demultiplexer in which the fourth invention isapplied to the second invention. The special feature of the presentembodiment is a structure which cancels the polarization dependence ofthe transmission center wavelength due to the waveguide birefringence ofeach of the input slab waveguide and the output slab waveguide byhalf-wave plates inserted in the input slab waveguide and the outputslab waveguide. A detailed description is given below with reference tothe drawings.

As shown in FIG. 8, the present array waveguide diffraction grating isan optical demultiplexer having seventeen input ports 41 and eightyoutput ports 22. The input slab waveguide 14 and the output slabwaveguide 18 are arranged in parallel, and a groove 42 having a width of18 μm and a depth of 200 μm is formed to intersect approximately themiddle of both slab waveguides perpendicular to the optical axesthereof. A half-wave plate 43 made of polyimide having a thickness of 15μm is inserted in the groove 42 at the position intersecting the inputslab waveguide so that the main axis is inclined at 45 degrees withrespect to the waveguide substrate 11, and a half-wave plate 45 made ofpolyimide having a thickness of 15 μm is inserted in the groove 42 atthe position intersecting the output slab waveguide so that the mainaxis is inclined at 45 degrees with respect to the waveguide substrate11, thereby forming each polarization mode converter.

In the present embodiment, the polarization dependence due to thewaveguide birefringence of the array waveguide 44 is canceled usingMethod {circle around (6)} described in the prior art examples. Namely,the array waveguide 44 is constructed by narrow-width channel waveguides44 a having a width of 6 μm and broad-width channel waveguides 44 bhaving a width of 14 μm, wherein the path length of the outsidenarrow-width channel waveguides 44 a become long, and the path length ofthe inside broad-width channel waveguides 44 b become long.

Further, the plurality of input ports 41-1 through 41-17 in the presentembodiment have a function which makes it possible to carry out fineadjustment of the transmission center wavelength of optical signals inaccordance with such input ports, and for that purpose the gap of theinput channel waveguides 32 connected to the input slab waveguide 14 ismade slightly wider than the gap of the output channel waveguide 21connected to the output slab waveguide 18 (Japanese Laid-Open PatentPublication No. HEI 8-211237). The output ports 22-17 to 22-80correspond to the input port 41-1, and this functions as an opticaldemultiplexer having a demultiplexing gap of 0.4 nm and a demultiplexingnumber of 64. The output ports 22-16 to 22-79 correspond to the inputport 41-2, and in the same way on downward, the output ports 22-1 to22-64 correspond to the input port 41-17. By selecting the input portsfrom this port arrangement, it is possible to carry out fine adjustmentof the transmission center wavelength of the demultiplexed opticalsignals by increments of 0.025 nm.

In an array waveguide having a plurality of both input ports and outputports like that of the present structure, the polarization wavelengthshift of the demultiplexed optical signals depend on each of the inputports and the output ports due to the effect of the waveguidebirefringence of the input slab waveguide 14 and the output slabwaveguide 18. Accordingly, in the array waveguide diffraction grating ofthe present embodiment, the common groove 42 is formed in approximatelythe middle of each of the input slab waveguide 14 and the output slabwaveguide 18, and polarization mode converters which depend on thehalf-wave plates 43, 45 made of polyimide and having a thickness of 15μm are inserted respectively in the groove 42 so that the main axes areinclined at 45 degrees with respect to the waveguide substrate 11, andthis canceled the polarization dependence of the transmission centerwavelength caused by the waveguide birefringence of the input slabwaveguide 14 and the output slab waveguide 18.

The same measurements as those of FIG. 6 were carried out for the arraywaveguide diffraction grating type optical demultiplexer of the presentembodiment, and the port dependence of the polarization wavelength shiftwas measured at each of the input ports and the output ports. As aresult, the port dependence of the polarization wavelength shift wasapproximately even and the polarization wavelength shift was suppressedto within ±0.005 nm for all the input ports.

From the above, it was confirmed that the array waveguide diffractiongrating type optical demultiplexer of the present embodiment operates asan optical demultiplexer which makes it possible to carry out fineadjustment of the demultiplexing wavelength without polarizationdependence of the transmission center wavelength.

As for the method of canceling the polarization dependence due to thewaveguide birefringence of the array waveguide 44, the array waveguidediffraction grating was manufactured by silica-based glass in whichdoping with germanium, silicon or boron was carried out above a doubleof the normal level in accordance with Method {circle around (5)}described in the prior art examples of the Embodiment 1, and this madeit possible to cancel the waveguide birefringence of the arraywaveguide. In this array waveguide diffraction grating, it wasunderstood that a waveguide birefringence of about 0.0004 nm was createdin the slab waveguides, and a polarization wavelength shift was createdin the output ports away from the center. Even in this array waveguidediffraction grating, it was confirmed that the output port dependence ofthe polarization wavelength shift in the slab waveguides was canceled bythe polarization mode converters provided in the slab waveguidesdescribed in the first through third embodiments.

Embodiment 4

FIG. 9 shows an embodiment of an array waveguide diffraction gratingtype optical multiplexer/demultiplexer according to the third invention.The special feature of the present embodiment is a structure which alsocancels the polarization dependence of the transmission centerwavelength due to the waveguide birefringence of the array waveguide inaddition to that of the output slab waveguide by a half-wave plateinserted in the output slab waveguide. A detailed description is givenbelow with reference to the drawings.

As shown in FIG. 9, the present array waveguide diffraction grating isgiven approximately the same structure as that of FIG. 4 except for thefact that there is no groove and half-wave plate in the array waveguide15 and the fact that a groove 51 is formed in the output slab waveguide18 at an incline with respect to the optical axis thereof and ahalf-wave plate 52 is inserted in the groove 51. Further, in the sameway as in the embodiment of FIG. 4, the half-wave plate 52 is made ofpolyimide and has a thickness of 15 μm, and is inserted and fixed by anadhesive in the groove 51 having a width of 18 μm and a depth of 200 μmwhich obliquely intersects approximately the middle of the output slabwaveguide 18 so that the main axis is inclined at 45 degrees withrespect to the waveguide substrate 11.

By properly selecting the angle θ formed between the groove 51 and aline A-A′ which is perpendicular to the optical axis of the output slabwaveguide 18, it is possible to collectively cancel the polarizationdependence of the transmission center wavelength due to the waveguidebirefringence of both the slab waveguide and the array waveguide. Themethod of calculating the incline angle θ will be described withreference to FIG. 10. As shown in FIG. 10, the array waveguide 15 isconstructed by many channel waveguides, and the conditions for carryingout interference and focusing inside the output slab waveguide 18 mustbe satisfied even between two channel waveguides. In this regard, adescription can be given using the two center channel waveguides. FIG.10 shows the state where the optical signals propagated in the TE modethrough the two mutually adjacent center channel waveguides C1, C2 ofthe array waveguide 15 are emitted respectively in the output slabwaveguide 18 having a focal length f at the connecting points P1, P2separated by a distance d, and are propagated the distances a1, a2 inthe TE mode to the half-wave plate 52 where conversion to the TM mode iscarried out at the points Q1, Q2, and then are propagated the distancesb1, b2 and focused at the point O on the center output port 22-3. Thechannel waveguide C1 is positioned outside from the center of the arraywaveguide 15 more than C2, and C1 is longer than C2 by the path lengthdifference ΔL.

If the difference of the optical path length (path length×refractiveindex) of the optical signals propagating respectively through these twopaths C1-P1-Q1-O and C2-P2-Q2-O is an integer m multiple of thewavelength λ(TE), because the optical signals will interfere and befocused at the point O, in the TE mode this gives the following:{na(TE)·ΔL+ns(TE)·a 1+ns(TM)·b 1}−{ns(TE)·a 2+ns(TM)·b 2}=m·λ(TE)  (6)

In the same way, in the case of the optical signals propagated throughC1, C2 in the TM mode, the wavelength is λ(TM), and this gives thefollowing:{na(TM)·ΔL+ns(TM)·a 1+ns(TE)·b 1}−{ns(TM)·a 2+ns(TE)·b2}=m·λ(TM)  (7)and the optical signals interfere and are focused at the point O. Fromthese two equations, when the conditions for making λ(TE)=λ(TM) tocancel the polarization dependence of the transmission center wavelengthare calculated, this gives the following:b 1−b 2=Ba·ΔL/(2Bs)  (8)

In this case, Ba is the waveguide birefringence of the array waveguide15, which is given by the following:Ba=na(TM)−na(TE)  (9)and Bs is the waveguide birefringence of the output slab waveguide 18given by Equation (5) described above. Further, the left side ofEquation (8) can be expressed using the incline angle θ of the half-waveplate 52 from geometric calculations, which gives the following:b 1−b 2=d·tan(θ)/2  (10)

In this case, an approximation is generally used based on the fact thatthe distance d between the two connecting points P1, P2 is sufficientlysmaller than the focal length f of the slab waveguide 18. As a result,from Equation (8) and Equation (10), the incline angle θ can becalculated by the following:θ=tan⁻¹ {Ba·ΔL/(Bs·d)}  (11)

In this case, the positive direction of θ is the rotation of thehalf-wave plate 52 in the counterclockwise direction in FIG. 9.

The method of inserting this wave plate at an incline in the slabwaveguide to also cancel the polarization dependence of the transmissioncenter wavelength due to the waveguide birefringence of the arraywaveguide is applied to an optical demultiplexer having a demultiplexinggap of 0.8 nm and a demultiplexing number of 64. In the array waveguidediffraction grating of the present embodiment, the demultiplexing gap istwice that of the previous embodiments, and the wavelength band requiredfor demultiplexing is also twice as wide. In order to obtain thesedemultiplexing characteristics, the path length difference ΔL of thearray waveguide 15 is made 16 μm, and the gap d of the array waveguide15 is made 15 μm at the connecting portion with the output slabwaveguide 18.

The results of measurements of the output port dependence of thepolarization wavelength shift when the half-wave plate 52 is notinserted are shown in FIG. 11. There is a polarization wavelength shiftof 0.2 nm even near the center output port number 32 due to thewaveguide birefringence Ba of the array waveguide 15, and it wasunderstood that the polarization wavelength shift due to the waveguidebirefringence Bs of the output slab waveguide 18 increases approximatelyin proportion to the output port number. From these results, eachwaveguide birefringence was calculated respectively to give Ba=0.0004,Bs=0.0007. Accordingly, when the incline angle θ of the half-wave plate52 required for canceling these polarization dependences was calculatedfrom Equation (11), this gave θ=31 degrees.

Next, the groove 51 was formed through the middle of the output slabwaveguide 18 at an incline of only 31 degrees counterclockwise from theline AA′ which is perpendicular to the optical axis of the output slabwaveguide 18. A polarization mode converter was constructed by insertingthe half-wave plate 52 in the groove 51. FIG. 12 shows the results ofmeasurements of the output port dependence of the polarizationwavelength shift after the insertion of the half-wave plate 52. It wasunderstood that it was possible to suppress the polarization wavelengthshift to within ±0.01 nm for all the output ports by the insertion ofthe half-wave plate 52.

From the above, it was confirmed that it is possible to cancel thepolarization dependence of the transmission center wavelength due to thewaveguide birefringence of the array waveguide in addition to that ofthe output slab waveguide by the half-wave plate inserted in the outputslab waveguide in the array waveguide diffraction grating type opticaldemultiplexer of the present embodiment.

Embodiment 5

FIG. 13 shows another embodiment of an array waveguide diffractiongrating type optical multiplexer/demultiplexer according to the thirdinvention. The special feature of the present embodiment is a structurewhich also cancels the polarization dependence of the transmissioncenter wavelength due to the waveguide birefringence of the arraywaveguide in addition to those of the input slab waveguide and theoutput slab waveguide by half-wave plates inserted in the input slabwaveguide and the output slab waveguide. A detailed description is givenbelow with reference to the drawings.

As shown in FIG. 13, the present array waveguide diffraction grating isgiven approximately the same structure as that of FIG. 9 except for thefact that there are many input ports and the fact that a groove 53 isformed in the input slab waveguide 14 at an incline with respect to theoptical axis thereof and a half-wave plate 54 is inserted in the groove53.

In the same way as in the embodiment of FIG. 8, the present arraywaveguide diffraction grating is an optical multiplexer/demultiplexerhaving seventeen input ports 31 and eighty output ports 22. Theplurality of input channel waveguides 32 in the present embodiment havea function which makes it possible to carry out fine adjustment of thetransmission center wavelength of optical signals in accordance with theinput ports thereof, and for that purpose the gap of the input channelwaveguides 32 connected to the input slab waveguide 14 is made slightlywider than the gap of the output channel waveguide 21 connected to theoutput slab waveguide 18. The output ports 22-17 to 22-80 are used forthe input port 31-1, and this functions as an optical demultiplexerhaving a demultiplexing gap of 0.8 nm and a demultiplexing number of 64.The output ports 22-16 to 22-79 correspond to the input port 31-2, andin the same way on downward, the output ports 22-1 to 22-64 correspondto the input port 17. This makes it possible to carry out fineadjustment of the transmission center wavelength of the optical signalsdemultiplexed between adjacent input ports by increments of 0.05 nm. Inthe present embodiment, in the same way as in the Embodiment 4, the pathlength difference ΔL of the array waveguide 15 is made 16 μm, and thegap d of the array waveguide 15 is made 15 μm at both the connectingportion between the array waveguide 15 and the input slab waveguide 14,and the connecting portion with the output slab waveguide 18.

The input port dependence and the output port dependence of thepolarization wavelength shift before insertion of the half-wave plates52, 54 in the array waveguide diffraction grating of the presentembodiment were measured, and this gave a distribution similar to thatof FIG. 11. As a result, it was understood that the waveguidebirefringence Ba was 0.0004 in the array waveguide 15, and the waveguidebirefringence Bs was 0.0007 in both the input slab waveguide 14 and theoutput slab waveguide 18. Then, grooves 51, 53 having a width of 18 μmand a depth of 200 μm were respectively formed in approximately themiddle of the input slab waveguide 14 and the output slab waveguide 18at inclines of only the angle θ/2 with respect to the lines AA′, BB′which are perpendicular to the optical axis of each slab waveguide. Inthis case, the positive direction of θ is the clockwise direction in theinput slab waveguide 14, and the counterclockwise direction in theoutput slab waveguide 18. The half-wave plate 52 made of polyimide isinserted into the groove 51, and the half-wave plate 54 made ofpolyimide is inserted into the groove 53 to construct polarization modeconverters. The reason why the incline angles of the half-wave plates52, 54 become half of the calculated angle θ described in Embodiment 4is because both half-wave plates 52, 54 cooperate to compensate thepolarization dependence of the transmission center wavelength due to thewaveguide birefringence of the array waveguide 15, and it is sufficientif the sum of the incline angles of the half-wave plates form θ given byEquation (11). The input port dependence and the output port dependenceof the polarization wavelength shift of the array waveguide diffractiongrating of the present embodiment were measured, and the polarizationwavelength shift was reduced to within ±0.01 nm for all the input ports.

From the above, it was confirmed that the array waveguide diffractiongrating of the present embodiment operates as an optical demultiplexerwhich makes it possible to carry out fine adjustment of thedemultiplexing wavelength without polarization dependence of thetransmission center wavelength.

Embodiment 6

FIG. 14 shows an embodiment of an array waveguide diffraction gratingtype optical multiplexer/demultiplexer in which the fourth invention isapplied to the third invention. The special feature of the presentembodiment is a structure which also cancels the polarization dependenceof the transmission center wavelength due to the waveguide birefringenceof the array waveguide in addition to those of the input slab waveguideand the output slab waveguide by half-wave plates inserted in one commongroove formed in the input slab waveguide and the output slab waveguide.A detailed description is given below with reference to the drawings.

The function of the array waveguide diffraction grating of the presentembodiment is approximately the same as that of Embodiment 5 shown inFIG. 13, but in the present embodiment, in order to make it possible toform one groove 55 into which half-wave plates 56, 57 are inserted, theinput slab 14 and the output slab 18 are arranged so that the anglesformed by extended lines of the optical axes thereof forms θ given byEquation (11). When given this kind of structure, it is possible toinsert the half-wave plates 57, 56 at inclines of the angle θ/2 from theline AA′ which is perpendicular to the optical axis of the input slabwaveguide 14, and the line BB′ which is perpendicular to the opticalaxis of the output slab waveguide 18. The waveguide birefringences Ba,Bs used the values calculated in Embodiment 5. This is because if thewaveguide structure and the manufacturing method are approximately thesame, then the waveguide birefringences will also have approximately thesame values.

The input port and output port dependences of the polarizationwavelength shift of the array waveguide diffraction grating of thepresent embodiment were measured, and the output port dependence of thepolarization wavelength shift was reduced to within ±0.01 nm for all theinput ports.

From the above, it was confirmed that the array waveguide diffractiongrating of the present embodiment cancels the polarization dependence ofthe transmission center wavelength due to the waveguide birefringence ofthe input slab waveguide, the output slab waveguide, and the arraywaveguide by the two half-wave plates inserted in the one groove, andoperates satisfactorily as an optical demultiplexer which makes itpossible to carry out fine adjustment of the demultiplexing wavelength.

Embodiment 7

FIG. 15 shows an embodiment of an array waveguide diffraction gratingtype optical multiplexer/demultiplexer in which the fifth invention isapplied to the third invention. The special feature of the presentembodiment is a structure which also cancels the polarization dependenceof the transmission center wavelength due to the waveguide birefringenceof the array waveguide in addition to those of the input slab waveguideand the output slab waveguide by a common half-wave plate inserted inone common groove formed in the input slab waveguide and the output slabwaveguide. A detailed description is given below with reference to thedrawings.

The function of the array waveguide diffraction grating of the presentembodiment is the same as that of Embodiment 6 shown in FIG. 14. In thepresent embodiment, the input slab waveguide 14 and the output slabwaveguide 18 are arranged to intersect each other. When given thepresent structure, in addition to the fact that one groove 58 issufficient, results are obtained with one half-wave plate 59 whichcancels the polarization dependence of the transmission centerwavelength. In contrast with the gap d of the array waveguide 15 being15 μm at the connecting portion of the array waveguide 15 with the inputslab waveguide 14 or the output slab waveguide 18 in Embodiment 6, inthe present embodiment the gap d is reduced to 12 μm. For this reason,the angle θ becomes large, and in accordance with this, the intersectingangle of the input slab waveguide and the output slab waveguide can alsobe made large, and this makes it possible to easily manufacture thearray waveguide diffraction grating type optical demultiplexer.

The polarization wavelength shifts of the output ports were measured forthe input ports in the array waveguide diffraction grating of thepresent embodiment, and the polarization wavelength shift was within±0.01 nm for all the input ports.

From the above, it was confirmed that the array waveguide diffractiongrating of the present embodiment makes it possible to also cancel thepolarization dependence of the transmission center wavelength due to thewaveguide birefringence of the array waveguide in addition to those ofthe input slab waveguide and the output slab waveguide by one commonhalf-wave plate inserted in the one common groove formed in the inputslab waveguide and the output slab waveguide, and can operatesatisfactorily as an optical demultiplexer which makes it possible tocarry out fine adjustment of the demultiplexing wavelength.

The polarization mode converter according to the sixth invention wasdescribed in each of the embodiments. Further, in the embodimentsdescribed above, a polarization mode converter which depends on ahalf-wave plate made of polyimide was used, but the polarization modeconverter is not limited to this, and may be a half-wave plate whichuses crystal or another anisotropic material, or the polarization modeconverter may be constructed by a stress imparting film such asamorphous silicon or the like formed on the waveguides.

The embodiments of an array waveguide diffraction grating type opticalmultiplexer/demultiplexer of the present invention were described aboveas examples of an optical multiplexer and an optical demultiplexer, butthe array waveguide diffraction grating of the present invention can beapplied to a wavelength router which is an opticalmultiplexer/demultiplexer having a plurality of input ports and aplurality of output ports, and which has a wavelength routing functionbetween the input ports and the output ports.

Further, the array waveguide diffraction grating used in the presentinvention is not limited to being formed on a silicon substrate, and canbe formed on silica glass, ceramic, plastic, or another semiconductorsubstrate. Further, the waveguide material is not limited tosilica-based glass, and it is possible to construct a waveguide by anoptical material such as a glass having other components, a plastic, asemiconductor or the like.

1. An array waveguide diffraction grating type opticalmultiplexer/demultiplexer, comprising: at least one input channelwaveguide, an input slab waveguide connected to said input channelwaveguide, an array waveguide formed from a plurality of channelwaveguides connected to said input slab waveguide, an output slabwaveguide connected to said array waveguide, and at least one outputchannel waveguide connected to said output slab waveguide; wherein apolarization mode converter is arranged in at least one of said inputslab waveguide and said output slab waveguide.
 2. The array waveguidediffraction grating type optical multiplexer/demultiplexer described inclaim 1, wherein the array waveguide formed from a plurality of channelwaveguides carries out means for canceling the polarization dependencedue to the waveguide birefringence.
 3. The array waveguide diffractiongrating type optical multiplexer/demultiplexer described in claim 1,wherein said polarization mode converter is a means for canceling thepolarization dependence due to the waveguide birefringence of said arraywaveguide, and the polarization dependence due to the waveguidebirefringence of at least one of said input slab waveguide and saidoutput slab waveguide.
 4. The array waveguide diffraction grating typeoptical multiplexer/demultiplexer described in any one of claim 1, claim2 or claim 3, wherein said polarization mode converter is provided in acommon groove formed in said input slab waveguide and said output slabwaveguide.
 5. The array waveguide diffraction grating type opticalmultiplexer/demultiplexer described in any one of claim 1, claim 2 orclaim 3, wherein said polarization mode converter is provided in commonin said input slab waveguide and said output slab waveguide in anintersecting portion formed by intersecting said input slab waveguideand said output slab waveguide.
 6. The array waveguide diffractiongrating type optical multiplexer/demultiplexer described in any one ofclaim 1, claim 2 or claim 3, wherein said polarization mode converter isa half-wave plate having a main axis inclined at 45 degrees with respectto said waveguide substrate.
 7. The array waveguide diffraction gratingtype optical multiplexer/demultiplexer described in claim 4, whereinsaid polarization mode converter is a half-wave plate having a main axisinclined at 45 degrees with respect to said waveguide substrate.
 8. Thearray waveguide diffraction grating type opticalmultiplexer/demultiplexer described in claim 5, wherein saidpolarization mode converter is a half-wave plate having a main axisinclined at 45 degrees with respect to said waveguide substrate.